Thickness dependence of the martensitic transformation, magnetism, and magnetoresistance in epitaxial Ni-Mn-Sn ultrathin films

We investigate the influence of the film thickness on the martensitic transformation for the example of Ni-Mn-Sn thin films. Epitaxial films with thicknesses ranging from 100 nm down to 10 nm were deposited on MgO by co-sputtering on heated substrates. The martensitic transformation is investigated using temperature dependent x-ray diffraction, magnetization and resistivity measurements. X-ray diffraction and transmission electron microscopy is used to study the growth and the martensitic structure of the films. We find that the martensitic transformation temperatures reduce and the transformation range increases with decreasing film thickness. We show that the transformation is still possible down to a film thickness of 10 nm. A systematic study on the resistance change caused by the martensitic transformation implies that the transformation is suppressed close to the interfaces to the MgO.

Figure 1

(a) The XRD patterns of the 100 nm thick Ni${}_{51.6}$Mn${}_{34.9}$Sn${}_{13.5}$ films at different tilt angles. The reciprocal space map for a commensurate 4O phase is shown in (b). The arrows marked by S${}_{-1}$ to S${}_2$ show schematically the $2\Theta$ scans. In (c) the superlattice peaks of the 4O/10M phase and the expected commensurate positions are presented by red solid lines/black dashed lines, respectively.

Figure 2

Pole figure measurements of the orthorhombic peaks and selected superstructure peaks. The fourfold symmetry verifies epitaxial growth on the MgO substrate.

Figure 3

The XRD signal of the structural transformation of the a Ni${}_{51.6}$Mn${}_{34.9}$Sn${}_{13.5}$ 100 nm thin film is shown in (a). The normalized integrated intensity of all peaks is plotted in (b). In (c) the temperature dependence of the lattice constants is shown. The temperature dependence of the $\gamma$ parameter is presented in (d). The sample is measured on the cooling branch of the thermal hysteresis.

Figure 4

A TEM image along the ${\left[011\right]}_{\text{A}}$ zone axis of the 100 nm thick Ni-Mn-Sn film at room temperature is shown. A FFT of the region where the nanotwins are visible is shown in the inset. The arrows indicate the features due to the periodicity of the nanotwins.

Figure 5

The XRD patterns of the Ni${}_{51.6}$Mn${}_{34.9}$Sn${}_{13.5}$ ultrathin films with different thicknesses at room temperature is shown in (a). The red bars show the Bragg positions of the cubic phase. The dashed lines indicate the shoulder positions. The evolution of the lattice parameter with the film thickness is shown in (b). In (c) the (110) reflection of Ni-Mn-Sn is shown.

Figure 6

The temperature dependence of FC and FH magnetization curves for ${\text{Ni}}_{51.6}{\text{Mn}}_{34.9}{\text{Sn}}_{13.5}$ thin films with different thicknesses [(a) 10, (b) 20, (c) 35, 50, 100 nm]. The arrows show the temperature change direction.

Figure 7

(a) The resistance change over the measured temperature range is plotted for Ni${}_{51.6}$Mn${}_{34.9}$Sn${}_{13.5}$ thin films with different thicknesses. The arrows indicate the temperature change direction. In (b) the measured maximal resistance change values for the films and the fit with Eq. (3) is shown. The structural and magnetic phase diagram is presented in (c), where $T_1$ is defined as $(M_{\text{s}}+A_{\text{f}})/2$ and $T_2$ as $(M_{\text{f}}+A_{\text{s}})/2$.

Figure 8

The temperature dependence of the magnetoresistance for a magnetic field of 1 T is plotted for Ni${}_{51.6}$Mn${}_{34.9}$Sn${}_{13.5}$ thin films with different thicknesses [(a) 10, 20, (c) 35, and (d) 50 nm]. The magnetic field dependence of magnetoresistance curves for 10 and 20 nm Ni${}_{51.6}$Mn${}_{34.9}$Sn${}_{13.5}$ thin films is shown in (b).